Third-party cross-site data sharing

Approaches, techniques, and mechanisms are disclosed for third-party tracking of user data. Instructions provided in association with a first document cause a client at which the first document is viewed to load a second document in a tracking document container associated with the first document. The second document belongs to a third-party domain that is different than a first domain to which the first document belongs. Instructions provided in association with the second document cause the client to retrieve a particular identifier from a local storage area for the third-party domain at the client. The instructions provided in association with the second document further cause the client to send a message to a server comprising the particular identifier.